A collection of units moving and attacking together. The engine creates autonomous unit groups to attack
polluted areas. The script can create and control such groups as well. Groups can accept commands in the
same manner as regular units.

add_member(unit)

Make a unit a member of this group. Has the same effect as giving a group_command with this
group to the unit.

Group position. This can have different meanings depending on the group state. When the group
is gathering, the position is the place of gathering. When the group is moving, the position is
the expected position of its members along the path. When the group is attacking, it is the
average position of its members.